Well, this problem may be quickly solved - but it lead me to some reflections on the topic asking myself why in Germany, where pretty much every food and beverage declaration is well specified this point is referred to the manufactures of the mineral waters, who in turn use it ambiguously.

According to this table "stilles Mineralwasser" has below 5.5 gram CO2 per liter.

The origin of "still" means, that it has no CO2 at all (no bubbling and no burping sound ;), but as always, the industry/marketing reinterpreted the term to their own means. So if you order/ask for a stilles Wasser, you ask for mineral water with little or no carbon dioxide. You will not get it more precise than that, because no one will measure how much CO2 is really in the bottle.

But the industry has an answer to the problem: "naturell" (see above). So first "still" meant "no CO2", then they redefined it and then someone really clever thought "they want no carbon dioxide, so they get no! (Or at least below 1 gram)" Welcome to the world of marketing.

The only distinction we made for non-fizzy water was sometimes mentioning the source "Hahnenwasser" (i.e. tap water).

Only later the term "Mineralwasser" for drinks came into fashion. This term was from where I lived until then reserved for water from a natural source of minerals (aka "Heilwasser"). There are spas where people can take a bath in naturally mineralized and sometimes even carbonized water.

Today however the term "Mineralwasser" is widely used for all kinds of water that you can drink. By German law there are clear definitions from what source water is allowed to be called Natürliches Mineralwasser, Quellwasser, or Tafelwasser but these definitions don't include the grade of carbonization. Also by this definition bottled water when named "Tafelwasser" is more or less identical to tap water. And, in some regions we may even have "Quellwasser" coming out of the taps.

For people's tastes several sub-levels of carbonization exist, but these are more or less defined by the phantasy of the water bottling industry. The most commonly understood may be:

Mineralwasser klassisch/classic: heavy carbonated water (Sprudel).

Mineralwasser medium: carbonated at an intermediate level.

Mineralwasser still: poorly or non-carbonated.

If you went to a restaurant and ordered a "stilles Wasser" you do however expect it to be not carbonated at all.